i found that the Sensor was vastly superior to the M3.
any time i visit my mate up in HM's neck of the woods we always pay a visit to Costco as i've to pick up M3 blades for the boys at work, among other things.
4 x 5 pack (20 cartridges) for less than £20
Costco is a kind of yank style cash 'n carry - you need a membership but the quality of stuff, especially the butcher section, and prices make it worthwhile
<!-- m --><a class="postlink" href="http://www.costco.co.uk/">http://www.costco.co.uk/</a><!-- m -->
and don't ever buy cartridge blades from ebay